Production of alcohols such as ethanol or butanol by fermentation with microorganisms is well known and has been industrially used since centuries. Historically, ethanol fermentation is the largest process. Acetone-butanol-ethanol (ABE) fermentation is considered as the second largest fermentation process [Duerre P: Production of solvents. In: Handbook on Clostridia, CRC Press, 2005: 671-696] and current production capacity in countries like China exceeds 1,000,000 tons [Ni Y and Sun Z: Recent progress on industrial fermentative production of acetone-butanol-ethanol by Clostridium acetobutylicum in China. Appl. Microbiol. Biotechnol., 2009, 83: 415-423]. Butanol is usually used as solvent or biofuel, while acetone is considered as an unwanted by-product [Duerre P: Production of solvents. In: Handbook on Clostridia, CRC Press, 2005: 671-696]. A few isolates of Clostridium beijerinckii are known to produce isopropanol instead of acetone due to a secondary alcohol dehydrogenase [George H A, Johnson J L, Moore W E C, Holdeman L V, Chen J S: Acetone, isopropanol, and butanol production by Clostridium beijerinckii (syn. Clostridium butylicum) and Clostridium aurantibutyricum. Appl Environ Microbiol 45: 1160-1163]. Isopropanol has similar properties as butanol and would be beneficial over acetone. However, the reduction is not very efficient and respective Clostridium beijerinckii strains don't produce good titers and are not considered as useful production strains.
Recently, C. acetobutylicum has been metabolically engineered for isopropanol production using a secondary alcohol dehydrogenase from C. beijerinckii [Lee et al, 2012: Metabolic engineering of Clostridium acetobutylicum ATCC824 for isopropanol-butanol-ethanol fermentation, Appl. Environ. Microbiol. 78: 1416-1423], but a highly efficient alcohol dehydrogenase would be required to optimize this process.
A challenge of the ABE fermentation is that all known organisms rely on sugar or starch based substrates. The cost of many carbohydrate feed stocks suitable for the production of chemical products such as acetone and isopropanol is influenced by their value as human food or animal feed, and the cultivation of starch or sucrose-producing crops for such production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into useful chemical products such as acetone and isopropanol.
CO is a major free energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually. Acetogenic organisms such as the closely related microorganisms Clostridium autoethanogenum, C. ljungdahlii, and C. ragsdalei are able to grow chemoautotrophically on CO or CO2/H2 containing gases as sole energy and carbon source and synthesize products such as acetate, ethanol, or 2,3-butanediol, but neither acetone nor isopropanol [Munasinghe P C, Khanal S K: Biomass-derived syngas fermentation into biofuels: Opportunities and challenges. Bioresource Technol 2010, 5013-22].
Recently, production of isopropanol was reported in a study on Clostridium ragsdalei (Clostridium strain P11) in a 100-L pilot scale fermentor from switchgrass derived syngas [Kundiyana D K, Huhnke R L, Wilkins M R: Syngas fermentation in a 100-L pilot scale fermentor: Design and process considerations. J Biosci Bioeng 2010, 109: 492-498]. However, related studies from the same lab showed that this was due to a contamination in the used syngas since it was passed through a scrubbing mixture containing 20% acetone [Ramachandriya K D: Effect of biomass generated producer gas, methane and physical parameters on producer gas fermentations by Clostridium strain P11. Masters thesis, Oklahoma State University 2009; Ramachandriya K D, Wilkins M R, Delorme M J M, Zhu X, Kundiyana D K, Atiyeh H K, Huhnke R L: Reduction of acetone to isopropanol using producer gas fermenting microbes. Biofuels Environ Biotechnol, 2011, epub]. The authors however confirmed acetone to isopropanol reduction by Clostridium ragsdalei (Clostridium strain P11) and speculated about the presence of a secondary alcohol dehydrogenase but couldn't find any evidence.
It is an object of the invention to overcome one or more of the disadvantages of the prior art, or to at least to provide the public with a useful choice.